BMI, Calories, Body Fat — Answered

BMI (Body Mass Index) is calculated as weight (kg) ÷ height (m)². The standard categories from the CDC:

BMI Range	Category
Below 18.5	Underweight
18.5 – 24.9	✓ Healthy weight
25.0 – 29.9	Overweight
30.0 and above	Obese

BMI is a quick screening tool, but it has real limitations — it doesn't distinguish between muscle and fat, doesn't account for age or sex differences in body composition, and can misclassify athletes (high BMI but low body fat) and older adults (normal BMI but high body fat).

BMI is a useful population-level screening tool, but it has significant limitations for individuals:

It can be wrong for:
• Athletes and muscular people — muscle is denser than fat, so a very fit person may have a "overweight" BMI despite low body fat
• Older adults — muscle loss with age means body fat is higher than BMI suggests
• People of different ethnicities — research suggests different BMI cutoffs may be appropriate for Asian, Black, and Hispanic populations
• Children — age-adjusted percentile charts are used instead of adult cutoffs

Better measures of metabolic health: waist circumference, waist-to-hip ratio, body fat percentage, and blood work. Use BMI as one data point, not the whole picture.

Weight loss requires a calorie deficit — eating fewer calories than you burn. One pound of fat ≈ 3,500 calories.

To lose 1 lb/week: Create a 500 calorie/day deficit
To lose 2 lbs/week: Create a 1,000 calorie/day deficit (upper safe limit for most people)

Start with your TDEE (Total Daily Energy Expenditure) — how many calories your body burns per day. Then subtract your target deficit.

Example: TDEE = 2,200 calories. To lose 1 lb/week, eat 1,700 calories/day.

Use our calorie calculator and TDEE calculator to get your personal numbers.

TDEE (Total Daily Energy Expenditure) is the total number of calories your body burns in a day — your metabolism plus all physical activity.

It's calculated in two steps:
1. BMR (Basal Metabolic Rate): Calories burned at rest (breathing, circulation, organ function) — using the Mifflin-St Jeor formula
2. Activity multiplier: BMR × your activity level

Activity multipliers:
• Sedentary (desk job, no exercise): BMR × 1.2
• Lightly active (1–3 days/week): BMR × 1.375
• Moderately active (3–5 days/week): BMR × 1.55
• Very active (6–7 days/week): BMR × 1.725
• Extra active (physical job + exercise): BMR × 1.9

Your TDEE is your maintenance calorie level — eat below it to lose weight, above it to gain.

Most health professionals recommend losing 0.5 to 2 pounds per week as a sustainable, safe rate.

Faster weight loss is possible but risks:
• Muscle loss (your body burns muscle when calories are very low)
• Nutritional deficiencies
• Metabolic adaptation (your metabolism slows)
• Rebound weight gain when normal eating resumes

The rule of thumb: Don't eat fewer than 1,200 calories/day (women) or 1,500 calories/day (men) without medical supervision.

Slower loss means more of what you lose is actually fat — and habits built gradually tend to stick. Most people who lose weight quickly regain it within a year.

The most accessible method is the US Navy formula, using body measurements:

Men: Measure waist (at navel) and neck. Formula uses these plus height.
Women: Measure waist (narrowest point), hips (widest point), and neck. Formula uses these plus height.

Healthy body fat ranges:
• Men: Essential fat 2–5% · Athlete 6–13% · Fit 14–17% · Average 18–24% · Obese 25%+
• Women: Essential fat 10–13% · Athlete 14–20% · Fit 21–24% · Average 25–31% · Obese 32%+

Use our body fat calculator to get your estimate from measurements.